1. Field of the Invention
The invention relates to a torque detector including magnetic yokes, and an electric power steering system including the torque detector.
2. Description of the Related Art
With reference to FIG. 15, the configuration of a conventional torque detector 200 will be described below. The torque detector 200 includes a permanent magnet 210, a pair of magnetic yokes 220 (220A, 220B), and a magnetic sensor 230. The permanent magnet 210 is magnetized such that the north poles and the south poles are alternately arranged in the circumferential direction. The magnetic yokes 220 are disposed so as to surround the permanent magnet 210. The first magnetic yoke 220A has a first ring 221 having an annular shape, and a plurality of first teeth 222. The second magnetic yoke 220B has a second ring 223 having an annular shape, and a plurality of second teeth 224. The magnetic sensor 230 is located between the first ring 221 and the second ring 223. The magnetic sensor 230 detects a magnetic flux density between the first ring 221 and the second ring 223. Japanese Patent No. 4561748 describes an example of a conventional torque detector.
If the area in which the permanent magnet and the teeth of each magnetic yoke are opposed to each other (hereinafter, referred to as “opposing area between the permanent magnet and the teeth of the magnetic yoke”) is increased, the amount of magnetic flux transferred between the permanent magnet and the teeth is increased and thus the density of magnetic flux passing through the magnetic sensor is increased. As a result, the accuracy of detection by the torque detector is enhanced. In the conventional torque detector 200, the opposing area between the permanent magnet 210 and the teeth 222, 223 can be increased by setting the width HR of each of the teeth 222, 224 to a larger value.
However, if the width HR of each of the teeth 222, 224 is set to a larger value, the gaps between the teeth 222 and the teeth 224 that are adjacent to each other in the circumferential direction may become excessively narrower, and, as a result, the amount of magnetic flux leakage between the first teeth 222 and the second teeth 224 is increased. Thus, there are limits to increasing the opposing area between the permanent magnet and the teeth of the magnetic yokes by increasing the width of each of the teeth of the magnetic yokes.